Spectrometer with discharge limiting means

ABSTRACT

An inductively coupled plasma spectrometer including shielding/sampling means (1) located between a plasma torch (3) and an optical system (4) of the spectrometer, wherein said shielding/sampling (1) means is associated with an enclosure (9) for the plasma torch such that a relatively high independance path (10, 11) is established for limiting flow of electrical current between said shielding/sampling means (1) and said enclosure (9).

This invention relates to an inductively coupled plasma spectrometer.The invention is advantageously applied to an optical emissionspectrometer (ICP-OES) wherein the plasma torch and the optical systemof the spectrometer may be axially aligned and will be described hereinin that context. Nevertheless it is to be appreciated that it is notthereby limited to such applications.

In emission spectrometers of the above type, radio frequency (RF) energyis inductively coupled into a gas, such as for example argon, which iscaused to flow through the torch to generate a plasma discharge. Theplasma is used to atomise and excite a sample that is injected into theplasma to cause the emission of light at wave lengths which arecharacteristic of the atomic composition of the sample. The emittedlight is detected and measured to obtain an analysis of the sample.

Analytical detection limits are improved in optical emissionspectrometers in which the cloud of excited atoms generated in theplasma is viewed by an optical detection system of the spectrometeraxially along the central axis of the plasma torch rather thanperpendicular to that axis as in some known instruments. However anaxially aligned optical system needs to be protected from the heat andcontaminants in the plasma exhaust. This may be done by interposing ashield that includes a sampling or viewing port or orifice between theplasma "tail" and the entrance end of the optical system. Such a shieldis best if made of a conductive metal, as this allows it to be adapted,for example by the incorporation of a cooling system, to minimise damagewhich may be caused to the shield or its insulating support structure byheat from the plasma or hot gases leaving the plasma.

In inductively coupled plasma spectrometers the plasma acquires a radiofrequency potential because of capacitive coupling between the inductioncoil and the plasma. This potential can cause an electrical discharge tooccur from the plasma to the sampling shield, the possibility of whichis increased if the shield is made of a conductive metal.

An object of the present invention is to provide an inductively coupledplasma spectrometer in which the problem of an electrical dischargeoccurring between the plasma and the sampling shield is eliminated or atleast substantially ameliorated.

A similar discharge problem can occur between the plasma and a conecontaining a sampling orifice in inductively coupled plasma massspectrometers (ICP-MS). The problem may be addressed in ICP massspectrometers by reducing the potential difference between the plasmaand the sampling cone, for example by special induction coilarrangements as in U.S. Pat. Nos. 5,194,731 and 4,501,965 (RE 33386) orby biasing the cone as in U.S. Pat. No. 4,682,026. However, these arerelatively costly solutions which may be commercially viable for ICPmass spectrometers in circumstances where the arcing problem isespecially critical in these spectrometers. In other circumstances thepresent invention may offer a solution to the problem of arcing which isrealisable in a simple and cost effective manner in ICP-OES as well asin ICP-MS instruments.

Accordingly, the invention provides an inductively coupled plasmaspectrometer including shielding/sampling means located between a plasmatorch and an optical system of the spectrometer, wherein saidshielding/sampling means is associated with an enclosure for the plasmatorch such that a relatively high impedance path is established forlimiting flow of electrical current between said shielding/samplingmeans and said enclosure.

Flow of electrical current between the shielding/sampling means shouldbe limited below a level which would sustain a discharge.

It has been shown that isolating the shielding/sampling means from theenclosure by an insulating medium does not provide the requisite highimpedance as reactance of the capacitance between the shielding/samplingmeans and the enclosure at the frequency of the plasma RF source istypically too low.

Preferably the high impedance path includes a circuit that is resonantat the frequency of the RF supplied to the induction coil of the plasmatorch. More preferably the circuit includes an inductance chosen suchthat it and the capacitance between the shielding/sampling means and theplasma system enclosure will form a parallel resonant circuit at thefrequency of the RF supply. The circuit may also include a variableinductor or capacitor, for example a trimming capacitor, for tuning thecircuit. Preferably the inductance includes an air-cored inductor.

In a preferred embodiment of the present invention, the circuit forestablishing a high impedance path between the shielding/sampling meansand plasma system enclosure is provided by an air-cored inductor whichis also used to supply a coolant to the shielding/sampling means. Inthis embodiment, the inductor is formed from hollow conductive tubing,for example of silver plated copper, through which a coolant such as forexample water is supplied for circulation through ducting in orassociated with the shielding/sampling means before it passes out of thesystem via a suitable outlet.

A parallel resonant circuit as provided by the invention has a highimpedance at the resonant frequency which is given by the product of theinductive reactance of the inductor and the quality factor (Q) of thetuned circuit. For example, at a frequency of between 27 to 100 MHz,which is a typical for the RF supply to the induction coil of the plasmatorch, a Q in excess of 400 may be readily realised. This may establisha high impedance at the resonant frequency such as may substantiallyreduce a discharge current from the plasma in comparison to aspectrometer not having the parallel resonant circuit.

A preferred embodiment of the present invention will now be described,by way of example only, with reference to the accompanying drawing. Thedrawing is a schematic diagram showing the physical location andelectrical connection of a shielding/sampling cone 1 within aninductively coupled plasma optical emission spectrometer according to anembodiment of the present invention.

The invention, as exemplified in the drawing, includes a conductivemetal cone 1 having a viewing aperture 2, interposed between a plasmatorch 3 and an optical system 4. The torch 3, cone aperture 2 andoptical system 4 are aligned such that a cloud of excited atoms from asample, as generated in the plasma torch, is viewed axially along thecentral axis 5 of the torch rather than perpendicularly to that axis asin conventional systems. This axial viewing arrangement improvesanalytical detection limits of ICP-OES instruments because emissionsfrom the excited atoms are viewed more efficiently than is the case if aside view is taken. The optical viewing system and associated detectionand analytical componentry and circuits as such are generally the samein the axial system as in the conventional perpendicular viewing system.Such systems, componentry and circuits are known in the art and are thusnot described in detail herein.

Plasma torch 3 includes inlets 6 for supplying a plasma forming gas,which is preferably argon; and an induction coil 7 for inductivelycoupling RF energy, preferably at a free running frequency of 40 Mhz(nominal), into the gas flowing through the torch to generate a plasma.A suitable RF supply (not shown) is connected to induction coil 7. Asample may be injected axially through an inlet 8 into the torch foratomisation in the plasma.

In operation of the torch the plasma acquires a potential from theinduction coil 7 and capacitive coupling will occur between the cone 1and an enclosure for the plasma system, which enclosure is representedby the ground connection shown at 9. This capacitive coupling isrepresented by the capacitor referenced as 10. In accordance with theinvention, an air-cored inductor 11 is also connected between the cone 1and enclosure 9, its inductance being such that it forms a parallelresonant circuit with capacitor 10 at the frequency of the RF supply toinduction coil 7. The parallel resonant circuit 10-11 provides arelatively high impedance path between cone 1 and enclosure 9 which actsto suppress or limit flow of electrical current from the plasma suchthat a discharge to cone 1 is avoided.

A variable capacitor (not shown) may be connected across the inductor 11(in which case it may be in parallel with capacitor 10 or may replacecapacitor 10) for adjusting the total capacitance to allow the circuitto be tuned to resonance. Alternatively or additionally inductor 11 maybe constructed such that its inductance is adjustable for tuningpurposes. In one embodiment this may be achieved by deforming the hollowtubing from which the inductor is formed. A preferred construction maybe to manufacture inductor 11 to provide the correct inductance toresonate with capacitor 10 at the plasma torch operating frequency.

Cone 1 is preferably made of metal, for example nickel, and is in heatconducting relationship with a heat sink (not shown) for extracting heatfrom the cone. The heat sink may include a duct for passage of acoolant, preferably water, therethrough. Conveniently, the tubing forsupply of the coolant to the heat sink associated with cone 1 may becoiled so as to form inductor 11. Thus the invention offers a simple andcost effective means for suppressing arcing to the cone in that twofunctions may be served by one component.

A supply of argon gas may be directed to pass out of aperture 2 in cone1, as indicated by arrows 12, to give added protection for the opticalsystem 4 from the plasma exhaust.

It has been found that for a Varian Liberty Model 150 AX ICP-OESinstrument, an inductor 11 needs to have an inductance of about 440 nHfor realisation of the invention. This may be formed by suitably coilinga coolant inlet tube of for example silver plated copper and of about 4mm OD. In other instruments the inductance may range in value from about60 to 700 nH.

To facilitate ignition of the plasma, it may be necessary to providemeans by which the resonant circuit can be detuned from the frequency ofthe plasma RF source or alternatively the quality factor (Q) of thetuned circuit may be decreased. For example the frequency of theresonant circuit may be detuned so that it lies above or below thefrequency of the RF source. Depending on the quality factor of the tunedcircuit, this may have the effect of reducing substantially the value ofthe high impedance path between cone 1 and enclosure 9. The resonantfrequency can be decreased by switching additional tuning capacitanceacross inductor 11 to lower the resonant frequency. The value of thehigh impedance path between shielding/sampling cone 1 and enclosure 9may alternatively be reduced by reducing the quality factor of the tunedcircuit. The quality factor may be reduced by reducing the value ofinductor 11 or by increasing the resistance of the resonant circuit. Thelatter may be achieved by switching a suitable resistance acrossinductor 11, by temporarily connecting cone 1 to enclosure 9 or byproviding another inductor mutually coupled to an inductor forming partof the cone resonant circuit which can be short circuited by anappropriate switch.

The invention described herein is susceptible to variations,modifications and/or additions other than those specifically describedand it is to be understood that the invention includes all suchvariations, modifications and/or additions which fall within the spiritand scope of the above description.

I claim:
 1. An inductively coupled plasma spectrometer including shielding/sampling means located between a plasma torch having an enclosure and an optical system of the spectrometer, said shielding/sampling means being connected with said enclosure surrounding the plasma torch such that a relatively high impedance path is established for limiting flow of electrical current between said shielding/sampling means and said enclosure as compared with the flow of electrical current from said enclosure if directly connected to said shielding/sampling means.
 2. An inductively coupled plasma spectrometer according to claim 1 wherein said plasma torch is supplied with an RF frequency and said path includes a resonant circuit, said resonant circuit resonant at or near the said RF frequency.
 3. An inductively coupled plasma spectrometer according to claim 2 wherein said frequency is in the range of 27 to 100 Mhz.
 4. An inductively coupled plasma spectrometer according to claim 2 wherein said resonant circuit includes an inductance and a capacitance.
 5. An inductively coupled plasma spectrometer according to claim 4 wherein said inductance includes an air cored inductor.
 6. An inductively coupled plasma spectrometer according to claim 4 wherein said inductance and capacitance between said shielding/sampling means and said enclosure form a parallel resonant circuit at said frequency.
 7. An inductively coupled plasma spectrometer according to claim 2 wherein said resonant circuit includes a variable capacitor for tuning said resonant circuit.
 8. An inductively coupled plasma spectrometer according to claim 2 wherein said resonant circuit includes a variable inductor for tuning said circuit.
 9. An inductively coupled plasma spectrometer according to claim 4 wherein said inductance is hollow for passing a coolant to said shielding/sampling means.
 10. An inductively coupled spectrometer according to claim 2 wherein the quality factor (Q) of said circuit is at least
 400. 11. An inductively coupled plasma spectrometer according to claim 10 including means for reducing said quality factor to facilitate ignition of said plasma torch.
 12. An inductively coupled plasma spectrometer according to claim 10 including means for detuning said resonant circuit during ignition of said plasma torch.
 13. An inductively coupled plasma spectrometer according to claim 4 wherein said inductance has a value of substantially between 60 to 700 nH.
 14. An inductively coupled plasma spectrometer according to claim 1 wherein said shielding/sampling means includes a sampling orifice/port.
 15. An inductively coupled plasma spectrometer according to claim 1 wherein said spectrometer is an optical emission spectrometer. 